Until biomass conversion becomes more widely used, carbohydrate fermentation processes are destined to be a decreasingly important source of industrial ethanol. Beverage alcohol is produced from a great variety of sources, including grains, potatoes, and fruit, but fermentation-based industrial alcohol is almost entirely obtained by yeast fermentation of molasses. Molasses (50 percent sucrose residue from sugar processing) is diluted with water to approximately 15 percent and under slightly acidic conditions is fermented by yeast to give 14 percent ethanol. Fractional distillation of the solution yields the commercial product: 95 percent ethanol. Approximately two and one-half gallons of blackstrap molasses is needed to make a gallon of 190-proof ethanol. (Alcohol content is usually described in terms of its proof value, which is twice its ethanol percentage.)
Although ethylene hydration was known in the early part of the nineteenth century, it was not until 1929 that it became an industrial process. Today it is the dominant method of producing etha-nol. Ethylene, obtained from the thermal cracking of petroleum fractions or from natural gas separation processes, is treated with complex phosphoric acid-based catalysts at temperatures above 300 degrees Celsius and steam at pressures of thousands of pounds per square inch. The ethanol can be fractionally distilled, and the residual ethylene can be recycled. Ethylene can also be passed into concentrated sulfuric acid, and after hydrolysis the ethanol can be distilled from the diluted sulfuric acid.
The 95 percent alcohol produced by any of these methods can be converted to nearly pure (absolute) alcohol by removal of the water by azeotropic distillation using benzene or trichloroethylene. Trace amounts of the hazardous benzene or trichloro-ethylene remaining make the absolute alcohol undesirable for beverage purposes but useful for industrial purposes in which the 5 percent water interferes with use requirements.
William J. Wasserman
See also: Glycolysis and fermentation; Grains; Sugars.
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